Ringing control circuitry with shared ringing loop current detector

ABSTRACT

A time division switching system is disclosed having a plurality of telephone line circuits and common control circuits for switching calls over a time division communication bus. Ringing loop current detectors are shared by groups of the line circuits for detecting call answer signals during active ringing on a call and for controlling the line circuits to interrupt, or trip, the ringing upon a called party answer. Each group of the line circuits is permanently connected to a ringing current supply via an individual one of the loop current detectors. Each line circuit includes ringing control circuitry operated and released by the common control circuits for connecting ringing current to a called line during an active ringing interval and for disconnecting it during a silent interval of the ringing cycle. The circuitry is responsive to a detection of a called station answer signal by the shared loop current detector for immediately tripping ringing. Apparatus is also provided in the circuitry for sensing answer signals during the silent intervals and for generating switchhook status signals during both the silent and active ringing intervals.

BACKGROUND OF THE INVENTION

This invention relates to incoming call signaling facilities forcommunication systems and particularly to circuitry for controllingsingle and plural phase incoming call ringing through communication linecircuits. The invention further relates to line circuit ringingarrangements cooperating with time-shared call answer detection and ringtripping control circuitry for incoming call ringing over pathsindependent of a main communication switching network.

It has long been a common practice in the telephone industry to alert acalled station to an incoming call by ringing a bell of the telephoneinstrument. Typically, the ringing occurs in cycles, each of whichillustratively comprises an active internal of one-second durationfollowed by a three-second silent period. Such ringing is known in theart as noncoded ringing. The silent and active ringing intervals areslightly different for coded ringing which is widely used on multipartylines and involves multiburst of active ringing intervals. The cyclesfor both coded and noncoded ringing are recurrent until the incomingcall is answered or the call is abandoned. When the call is answered,ringing is terminated, or tripped, as soon as practicable to preclude atransmission of high level ringing signals through the telephonereceiver to the answering party.

A problem in many prior art telephone systems is that the call answerand ring tripping circuitry must be uniquely dedicated and assigned toeach called station for the entire duration of an incoming call eventhough it is only functional for a small percentage of the call holdingtime. Such circuitry is, in a substantial percentage of commerciallyoperating telephone systems, customarily integrated into a trunk circuitwhich supplies ringing power to a called line via a switching network.The trunk circuit generally receives the incoming call and controls theentire ringing operation.

Each such trunk circuit is generally equipped with apparatus foractivating one of a group of ringing selection switches to select anappropriate code and phase of ringing power for periodically actuatingthe ringer of the called telephone. The call answer detection and ringtripping circuitry in each trunk circuit is frequently a relay, orequivalent device, which is connected to the called telephone line forthe recurrent cycles of silent and active ringing intervals and forsensing D. C. loop current flow over the line resultant from a calledparty answer. Upon detecting such loop current, the ring trip circuitryis effective to interrupt further ringing of the called station.Thereafter, the call answer and ring trip circuitry remains inactive andout-of-service for the remainder of the call. The inactivity andout-of-service factors obviously result in inefficient uses of callanswer and ring trip circuitry.

In an endeavor to overcome the inefficient dedication of call answerdetection and ring tripping circuitry, the art progressed in recentyears to the extent that such circuitry is sometimes divorced from trunkcircuits and segregated into ringing control circuits. Each of thelatter circuits is switchable onto call connections to perform theentire ringing control job including the detection of a called partyanswer and the ring trip control functions. Importantly, after thesefunctions are completed, the control circuit is automatically releasedfrom call connections before call conversation commences and is thenmade available for serving another call. Such an arrangement isdisclosed, for example, in L. F. Goeller, Jr. U.S. Pat. NO. 3,378,650,issued Apr. 16, 1968.

A disadvantage of the Goeller arrangement is that the common controlequipment must establish several distinct connections through aswitching network. One such connection is to a ringing control circuitfor the ringing job. A subsequent connection is to a reserved trunkcircuit for call conversation. Such an approach requires many commoncontrol equipment work operations to hunt, test and establish thenetwork connections and undesirably requires separate network channelallocations especially for the ringing job.

The need for the latter work operations and special ringing channelallocations has been reduced by another innovation in the art asdisclosed in A. Feiner, U.S. Pat. No. 3,492,437 of Jan. 27, 1970.Advantageously, Feiner discloses a ringing control circuit which is timeshared by a plurality of trunk circuits and which is connectable ontothe same network connections as the trunk circuits for performing theringing job. The Feiner innovation enables a call answer detector andring trip control circuit to be shared among a plurality of trunkcircuits during active ringing which is time spaced for each sharingcircuit. Each such ringing control circuit is equipped with connectorrelays which are operated by a switching network controller to connectactive ringing power and a shared called station answer detector througha trunk and line link network to a called station.

While the foregoing Feiner arrangement has proven technically reliablefor many commercial applications, it teaches that each ringing controlcircuit be associated with trunk circuit and that it be controlled by aswitching network controller for performing the ringing job overconnections through the switching network. In certain present dayswitching systems, such as time division systems, the foregoing Feinerteaching does not find practical utility because it is not usuallyfeasible to switch ringing current from trunk circuit appearances in thenetwork through time slot switching elements to called lines. Moreoever,the individual Feiner ringing control circuits require circuitry whichself-determines the active ringing interval and the disconnection ofringing from a call connection through the network. Suchself-determinations are made by a timer arrangement in the ringingcontrol circuits.

Another Feiner invention disclosed in A. Feiner (Case 32), U.S. patentapplication Ser. No. 521,734, filed concurrently herewith, sets forthringing control circuitry which utilizes the shared ringing loop currentdetector technique for single and multiphase ringing and whichadvantageously does not require the switching of ringing power throughthe communication switching network. It is therefore suitable for use intime division systems and other such systems utilizing low levelelements for communication switching. In the latter Feiner arrangement,a group of the line circuits time share a single call answer detectorwhich is connectable and disconnectable from called lines via the linecircuits in response to signals from common control equipment.Advantageously, the duration of each connection of the answer detectorto a called line is determined soley by the common control equipment fora full active ringing interval. This centralized determinationeliminates the necessity of circuitry in individual line circuits forthe same function and reduces the need for prior art common controlcircuit and scanner interrogation of the line circuits to determinewhether the answer detector has been disconnected from a called lineupon termination of active ringing.

Each of the line circuits is illustratively connected permanently to theshared detector and is equipped to connect it to a called line at thebeginning of an active ringing interval and to disconnect the detectorfrom the line at the end of that interval and during the entire silentinterval. The Feiner arrangement advantageously is also equipped tobridge a line supervisory relay onto the called line for sensing acalled station answer during the silent interval and thereupon tocontrol the tripping of ringing.

In view of the foregoing, there has existed a recognized need in the artfor improved arrangements for line circuits to control the ringingfunctions in systems utilizing shared ringing loop current detectors andin which the ringing is switched over paths independent of the maincommunication switching network.

SUMMARY OF THE INVENTION

The foregoing need is fulfilled in accordance with an illustrativeembodiment of our invention which provides improved ringing controlcircuitry for line circuits for use, by way of example, in a timedivision switching system. The illustrative embodiment utilizes aringing loop current detector which is time shared by a plurality ofline circuits as in the ringing arrangements disclosed in theaforementioned Feiner patent application.

Each one of the line circuits comprises switch means which is operablefor connecting ringing power from a shared one of the detectors throughthat one line circuit to a called line and which is releasable fordisconnecting the ringing power from that line. The line circuit isadvantageously equipped with control means responsive to a receipt ofcontrol signals supplied by common control equipment for operating theswitch means during each active ringing interval and releasing theswitch means during each silent interval.

It is a feature of our invention that the switch means includes anelectromechanical switch and the control means includes a bistablecircuit arrangement responsive to control signals from the controlequipment for controlling the operation and release of theelectromechanical switch.

According to another aspect of our invention, the electromechanicalswitch comprises a ring relay and the bistable circuit arrangementincludes a flip-flop circuit settable for operating the relay to connectringing power from the shared one of the detectorsto the called line andresettable for releasing the relay to disconnect the ringing power fromthe line. The circuit arrangement further includes logic means includinga line circuit select gate together with set and clear gates forcontrolling the setting and resetting of the flip-flop circuit.

An important feature of our invention is that the flip-flop circuitfurther includes additional means for resetting the flip-flop circuitimmediately upon detection by the shared detector of an answer signalfrom the called line and during a connection of the ringing power tothat line. This resetting operation causes the immediate release of thering relay for disconnecting active ringing power from the called lineand thereby ensures that the called party does not receive objectionableringing signal through the telephone receiver. An advantage of thisresetting arrangement is that the shared detector provides a commonreset signal to all flip-flop circuits in the same group of linecircuits.

Another feature is that the ring relay is effective to connect a linesupervisory relay to a called line during each silent interval of aringing phase. The supervisory relay illustratively senses loop currentas a call answer signal and then generates a call answer status signalfor informing the common control equipment to terminate further activeringing operations.

DESCRIPTION OF THE DRAWING

The foregoing features and advantages, as well as others, of thisinvention can be more fully understood from a reading of the followingdescription with reference to the drawing in which:

FIG. 1 is a block diagram of a time division switching system employingringing loop current detectors shared by groups of telephone linecircuits;

FIG. 2 is a sequence diagram which depicts four illustrative phases ofringing assigned by a time division processor for operating the ringingcircuitry of FIG. 2; and

FIG. 3 shows schematically a ringing generator and a shared ringing loopcurrent detector connected to a single line circuit having ringingcontrol circuitry, as well as an active hydrid for TDM callcommunications.

The equipment illustrative of the principles of this invention has beendesigned for incorporation, by way of example, into a program controlledpulse amplitude modulation time division switching system of the typeessentially as disclosed in the D. J. H. Knollman U.S. Pat. No.3,914,559 issued Oct. 21, 1975 and assigned to the same assignee.Related subject matter is also disclosed in A. Feiner U.S. patentapplication Ser. No. 521,734 filed Nov. 7, 1974 concurrently herewith,and J. F. O'Neill U.S. Pat. 3,916,118 issued Oct. 18, 1975 and assignedto the same assignee. It is particularly concerned with the sharedringing loop current detector, the line circuit ringing controlcircuitry and a ring relay switching arrangement associated with theline circuit hybrid. The other equipment elements are neither shown nordescribed in detail herein, except where necessary for a completeunderstanding of the construction and operations of our illustrativeringing equipment. Reference is also made to the J. M. Elder, Jr., U.S.Pat. No. 3,934,099 issued Jan. 20, 1976 and assigned to the sameassignee, which discloses details of the hybrid suitable for use in ourillustrative telephone system.

DETAILED DESCRIPTION

In FIG. 1, the illustrative TDM system comprises a plurality oftelephone stations TS1-TS128 each of which is equipped with aconventional telephone ringer (not shown). The stations are connectedover respective telephone lines L1-L128 to TDM line circuits LC1-LC128.

According to our exemplary embodiment, the line circuits are dividedinto four line groups LG1-LG4 each of which comprises 32 line circuits.Illustratively, line group LG1 comprises the line circuits LC1-LC32 andgroup LG4 comprises circuits LC97-LC128. Each of the line circuits isconnected to a time division communication and control bus system and isequipped with circuitry which is activated over the bus system by commoncontrol circuits CC for controlling incoming telephone call ringing andthe establishment, supervision and termination of time slot switchconnections for transmit and receive TDM communication on such calls.

Ringing current for alerting a called one of the stations TS1-TS128 toan incoming call is, by way of example, supplied to the line circuitsLC1-LC128 from a common ringing generator RG. It continuously furnishesnominal 20 cycle, 120 volts superimposed on negative 48 volts frombattery B. Generator RG is advantageously permanently connected via fourringing loop current detectors D1-D4 to the line circuits in therespective line groups LG1-LG4. Each one of the detectors D1-D4 isindividual to one of the groups LG1-LG4 and is shared by all linecircuits in that one group for sensing during ringing a loop currentflow resultant from a called station answering (telephone off-hook) ofan incoming call. To elaborate, detector D1 is shared by line circuitsLC1-LC32 and detector D4 is shared by circuits LC97-LC128.Advantageously, each of the detectors D1-D4 is utilized for sensing loopcurrent for four different phases of ringing. Detectors D1-D4 areconnected to the line circuits of the respective line groups via theleads RSG1-RSG4.

Each of the line circuits LC1-LC128 is further equiped according to ourinvention with ringing control circuitry (FIG. 3) which is responsive tosignals received via the bus system from the common control circuits CCfor connecting and disconnecting active ringing current to and from arespective called one of the lines L1-L128. The control circuit CCadvantageously partitions the operation of the ringing circuitry for aline circuit to provide four phases of ringing. As depicted in FIG. 2,the four phases A-D each comprise a cyclically recurrent one-secondactive period of ringing followed by a three-second silent intervalunder control of the control circuit CC. The active period of ringing ineach of the phases A-D is time spaced and arranged uniquely to occurwhile the other three phases are in respective silent intervals as shownin FIG. 2.

Our exemplary embodiment permits a single ringing loop current detectorD1 to be shared among all lines L1-L32 within line group LG1 becauseonly one such line is actively rung at a time during an assigned one ofthe ringing phases A-D. The detector is operable during the assignedphase to detect an answer signal from only the one line being rung. Ourringing system is further arranged so that illustratively a maximum offour different lines in the same line group, such as group LG1, may besequentially rung during each of the successive phases A-D.Advantageously, the ringing loop current detector shared by those fourlines is sequentially connectable individually to those lines forsensing a called station answer signal during each of the active ringingintervals of all four phases A-D. Moreover, our ringing circuitry isdesigned so that one line in each of the line groups LG1-LG4 can beconcurrently rung during each of the phases A-D. Accordingly, in theillustrative embodiment, a maximum of four lines in each of the fourline groups LG1-LG4, that is a total of 16 different lines, may be rungin the four phases A-D, and one line in each line group, or a total offour lines, may be rung in each of the phases A-D.

In FIG. 3, the ringing arrangement for a single line group LG1 isdepicted utilizing a single line circuit LC1. The latter circuit isequipped with a ring relay RINGO which is oprable in response to signalsreceived over the system buses from the common control circuits CC forconnecting ringing current on lead RSG1 in any of the four phases A-Dvia contact RINGO-1 to the line L1 and station TS1. The control circuitsCC effect the operation of relay RINGO at the beginning of an activeone-second ringing interval of a selectably assigned one of the phasesA-D and causes its release to terminate that active interval and thenbegin a three-second silent interval for the same phase. This ringingoperation is recurrent until either the called station answers or theincoming call to that station is abandoned.

Referring to FIG. 3, a fundamental building block for the ringingcontrol circuitry of line circuit LC1 includes a ring flip-flop RFF. Itcomprises conventional inputs and outputs, namely: a set input S; a pairof clear, or reset, inputs C1 and C2; and "1" and "0" outputs.

Flip-flop RFF in its idle, or cleared, state rests with a logic 1 at its0 output and a logic 0 at its 1 output. As a consequence, relay RINGOwhich is connected to the 1 output of flip-flop RFF via an inverteramplifier REL, is released and withholds ringing power from line L1 atits contact RINGO-1.

When station TS1 is to be rung in response to a receipt of an incomingcall, the common control circuits CC select the appropriate one of thephases A-D and apply line selection and ringing control signals to thebus system for activating flip-flop RFF. The line selection and ringcontrol process involves a selection gate SEL, a ring bus set gate RBSand a ring bus clear gate RBC. Gate SEL is an AND gate which is inresponse to a receipt of coincident enabling LOW signals at its inputfrom select leads BPS* and BBS* for producing an output for partiallyenabling gates RBS and RBC. Concurrently, gate RBC, an inverting ANDgate, is responsive to a receipt of a disabling signal on lead BRBC forprecluding a subsequent clearing of flip-flop RFF by gate RBC during anactive interval of an assigned one of the ringing phases A-D. At thesame time, gate RBS, an inverting AND gate, is responsive to a receiptof an enabling signal on lead BRBS for producing an output signal whichfurther disables gate RBC and simultaneously sets flip-flop RFF forgenerating a logic 0 at its 0 output and a logic 1 at its 1 output. Thelatter action causes the operation of relay RINGO via an invertingamplifier REL. In operating, relay RINGO initiates an active interval ofringing on line L1 by closing its contact RINGO-1 to extend the ringingpower on lead RNG through a resistor RD of detector D1 and lead RSG1 tothe ring lead R of line L1 for ringing station TS1. Active ringingcontinues for the full one-second period unless the called stationanswers within that time.

If station TS1 does not answer the call before the expiration ofringing, the common control circuits CC control the deactivation of thering bus set gate RBS and the activation of the ring bus clear gate RBC.The control circuits do so by applying concurrent signals to the BRBSand BRBC input leads to gates RBS and RBC. The activation of gate RBCgenerates an output signal which is applied to the clear input C1 offlip-flop RFF for resetting the flip-flop. In resetting, flip-flop RFFswitches its 1 output to a logic 0 for effecting a release of relayRINGO via amplifier REL. Upon releasing, relay RINGO opens its contactRINGO-1 for terminating the active period of ringing on line L1 andentering a three-second silent period.

During the silent interval, the line circuit LC1 monitors the calledstation TS1 to detect a called party answer. Following such a detection,our exemplary ringing control circuitry signals the common controlcircuits CC to terminate further active ringing of station TS1 and toeffect further operations of line circuit LC1 for call communication inan available one of the TDM time slots.

The monitoring action is performed by a loop current supervisory relayLC0 which is connected across the tip and ring leads via contact RINGO-2and inductor LBF upon the aforementioned release of relay RINGO.Thereafter, relay LC0 is operative in response to current flow over thetip and ring loop resultant from the removal of the telephone handsetfrom its cradle and the switchhook contact closure. Upon operating,relay LC0 closes its contact LC0-1 for applying to an input of aninverting amplifier LC, a ground potential instead of the positivepotential via resistor R1. As a consequence, amplifier LC produces anoutput which enables a switchhook status AND gate SS. The enabling ofgate SS, as shown in FIG. 3, is controlled by coincident input signalsreceived from amplifier LC, the 0 output of the reset flip-flop RFF, andinverter amplifier BS. The latter gate produces an enabling outputsignal as a result of the aforementioned selection signal applied by thecommon control circuit CC to leads BBS* and BPS*. Upon enablement, gateSS generates an output signal on lead SSO for informing the commoncontrol circuits that the incoming call has been answered and activeringing is no longer needed for station TS1.

In the event that a called party answer signal is not detected withinthe silent period following an immediately previous active ringinginterval, the common control circuits CC effect the reoperation of relayRINGO, as already described, again to apply active ring current to lineL1 for a one-second interval. As priorly aexplained, the relay RINGO isoperated in response to the setting of flip-flop RFF and by its 1 outputsignal activating amplifier REL. Flip-flop RFF is set under control ofgates RBS and RBC which are respectively enabled and disabled byconcurrent signals received over leads BRBS and BRBC from the commoncontrol circuits.

According to our exemplary embodiment, when a called party answers atstation TS1 during the active ringing interval, ringing is tripped andthereby the active ringing interval terminated under control of theshared ringing loop detector D1. The ring tripping action is speedilyperformed to preclude the transmission of ringing through the receiverof the telephone to the answering party. The latter detector senses achange in D.C. (direct current) loop current flow in the series pathfrom battery B, ringing generator RG, leads RNG and RSG1, contactRINGO-1 and the L1 line loop including station TS1.

As illustrated in FIG. 3, the detector comprises a ringing detectorresistor RD serially connected with leads RNG and RSG1. It also includesa low pass filter F which rejects, or blocks, 20 cycle ringing signalsfrom being applied to a differential amplifier DA and enables DC loopcurrent flow signals to be developed across resistor RD and passed toamplifier DA. Rejection of the 20 cycle signals is needed to precludeundesired ring tripping. Amplifier DA senses current changes developedacross resistor RD by loop current flow resultant from a called stationanswer. Amplifier DA then generates an output ring trip signal on leadRT. The latter signal immediately controls a resetting of flip-flop RFFand, in turn, the release of relay RINGO by being applied over lead RTdirectly to the clear input C2 of flip-flop RFF. As indicated in FIG. 3,lead RT is multiple connected to all of the corresponding clear inputsC2 of flip-flops RFF in all other line circuits LC2-LC32 in the sameline group which shares detector D1. However, as mentioned previously,only the flip-flop RFF is reset by the signal on lead RFF inasmuch asour illustrative ringing system only allows one lien circuit within thesame group to be rung within a selectably assigned one of the phasesA-D.

When relay RINGO releases for ring tripping, it opens its contactRINGO-1 to disconnect ringing current on lead RSG1 from line L1. At thesame time, relay RINGO closes its contact RINGO-2 and connects relay LCOacross the tip and ring leads for operation of that relay by the loopcurrent on line L1 caused by the closed switchhook of telephone TS1 uponthe called party answer. In operating, relay LC closes its contact LCO-1for activating amplifier LC and thereby enabling gate SS to supply aswitchhook status signal for station TS1 to lead SSO for informing thecommon control circuits that the incoming call has been answered. Thecommon control circuits thereupon proceed to establish TDM callconversation connections within an available time slot of the timedivision frame as described in the aforementioned O'Neill and Knollmanpatent applications.

It is to be understood that the hereinbefore described arrangements areillustrative of the application of principles of our invention. In lightof our teaching, it is apparent that numerous other arrangements may bedevised by those skilled in the art without departing from the spiritand scope of our invention.

What is claimed is:
 1. For use in a communication system having a groupof communication line circuits and incoming call signaling means,meansconnecting incoming call alerting signals from said signaling means tosaid line circuits and said means connecting being shared by saidcircuits for detecting call answer signals for each of said circuits,and each one of said line circuits having signaling control meansactivated in response to a receipt of control signals for connecting anincoming call alerting signal from said detecting means to a called linemeans subsequently responsive to said detecting means detection of ananswer signal on said called line for deactivating said control means todisconnect said alerting signal from said called line, and the inventionwherein said signaling control means in each one of said line circuitsincludes a switching device operable for connecting said incoming callalerting signal from said detecting means to said called line andbistable means activated in response to said control signals foroperating said switching device to connect said alerting signal to saidcelled line, and said detecting means being subsequently operated inresponse to said answer signal for deactivating said bistable means torelease said switching device and thereby to disconnect said alertingsignal from said called line.
 2. The invention of claim 1 wherein saidcontrol signals define active and silent periods of alerting signals,andsaid signaling control means further comprises logic means responsiveto a receipt of control signals for activating and deactivating saidbistable means to operate said switching device and thereby apply saidalerting signal for an active period of said call signaling and torelease said device for a silent period of said signaling.
 3. Theinvention of claim 2 wherein said signaling control means furthercompriseslogic apparatus operable in response to said control signalsfor activating said bistable device to operate said switching device andconnect said called line for said active period of said alertingsignaling and a logic device controlled by said logic apparatus andbeing responsive to said control signals for deactivating said bistablemeans to release said switching device to disconnect said alertingsignal from said line for said silent period of signaling.
 4. Theinvention of claim 3 wherein each one of said line circuits furthercomprises means jointly controlled by said bistable means, switchingdevice and said called line for generating a call answer status signal.5. The invention of claim 4 wherein said call answer status signalgenerating means comprisesmeans connectable to said called line uponeach said release of said switching device for sensing a call answersignal on said line, means controlled upon each said deactivating ofsaid bistable means and responsive to a sensing of a call answer signalby said sensing means for producing said call answer status signal. 6.The invention of claim 5 wherein each one of said line circuits furthercomprises means responsive to a receipt of signals selecting said one ofsaid line circuits for incoming call signaling for providing a signal toenable said logic apparatus and logic device to control said activatingand deactivating of said bistable means and further to enable said callanswer status signal producing means to produce said status signal. 7.For use in a communication system having a group of communication linecircuits and ringing power supply means,the invention comprising asingle ringing loop current detector permanently connected concurrentlyto each of said line circuits during ringing and communication intervalsand for detecting loop current flow over a called line during anapplication of ringing power from said supply means to said called line,means in each one of said line circuits responsive to a receipt ofcontrol signals for controlling a connection of said detector throughsaid one of said line circuits to a called line connectable thereto, andswitch means controlled by said controlling means for connecting saiddetector and said ringing power supply means to said called lineconnectable to said one of said line circuits.
 8. The invention inaccordance with claim 7 wherein said controlling means comprisesabistable circuit activatable in response to a receipt of ringing controlsignals for operating said switch means to connect said detector andsaid ringing power supply means serially with said called lineconnectable to said one of said line circuits and being furtherdeactivatable in response to ringing disconnect signals for releasingsaid switch means to disconnect said detector and said supply means fromsaid called line connectable to said one of said circuits, and logicmmeans responsive to said ringing control and disconnect signals foractivating and deactivating said bistable circuit, and wherein saidbistable circuit further includes means responsive to a receipt of asignal generated by said detector upon detection of loop current flowover said called line connectable to said one of said line circuits fordeactivating said bistable means.
 9. For a communication systemhavingplural groups of communication line circuits, ring current supplymeans, a plurality of call answer signal detectors connected to saidsupply means for extending ringing power to said line circuits andcommon control means for supplying signal to said line circuits forselectively controlling a generation of plural phases of ringing throughsaid line circuits to respective called lines, each of said phaseshaving an active ringing interval followed by a silent interval andbeing time spaced so that each said active ringing interval for eachsaid phase occurs during a silent interval of each of the other of saidphases, said controlling signals being supplied so that each said phaseis concurrently generatable for only one of said line circuits in eachof said groups, the improvement comprising said line circuits in eachone of said groups sharing an individual one of said detectors fordetecting incoming call answer signals, each one of said line circuitsincluding means operated in response to said supplied controllingsignals for connecting incoming call ringing power for one of saidphases from a respective shared one of said detectors through said oneof said line circuits to a called line, means subsequently activated bysaid shared one of said detectors upon a detection of a call answersignal received from said called line through said one of said circuitsduring an active ringing interval of said one of said phases forreleasing said connecting means to terminate said last-mentioned activeringing interval and wherein each of said detectors detects a loopcurrent call answer signal on a respective called line during an activeringing interval of each of said phases, and said connecting means ofeach one of said line circuits comprises switch means operable forconnecting ringing power from a respective shared one of said detectorsthrough said one of said line circuits to a called line and beingreleasable for disconnecting said ringing power from said last-mentionedline, and control means responsive to a receipt of said supplied controlsignls for operating said switch means during an active ringing intervalof one of said phases and releasing said switch means during a silentinterval of said last-mentioned one of said phases.
 10. The invention ofclaim 9 whereinsaid switch means includes an electromechanical switch,and said control means includes a bistable circuit arrangementresponsive to said receipt of said supplied signals for controlling theoperation and release of said electromechanical switch.
 11. Theinvention of claim 10 whereinsaid electromechanical switch comprises arelay, and said bistable circuit arrangement includes a flip-flopcircuit settable for operating said relay to connect said ringing powerfrom said shared one of said detectors to said called line and beingclearable for releasing said relay to disconnect said ringing power fromsaid called line, and logic means responsive to a receipt of saidsupplied control signals for controlling the setting and clearing ofsaid flip-flop circuit.
 12. The invention of claim 11 wherein said logicmeans includes set and clear gate means for controlling said setting andclearing of said flip-flop circuit.
 13. The invention of claim 12wherein said logic means further includes a gate circuit responsive to areceipt of line circuit select signals from said common control circuitsfor cooperating with said set and clear gate means to control saidsetting and clearing of said flip-flop circuit.
 14. The invention ofclaim 11 wherein said flip-flop circuit further includes means forclearing said flip-flop circuit immediately upon a detection of ananswer signal from said called line during a connection of said ringingpower thereto.